FIG. 1 is a block diagram of a system, indicated generally by the reference numeral 1, that illustrates some of the problems that the present invention seeks to address.
The system 1 comprises a DC source 2, an inverter 4, a PWM controller 6, a paralleling device 8 and an AC grid 10.
The DC source 2 provides DC power to the inverter 4. As is well known the art, the inverter 4 can be switched under the control of the PWM controller 6 to generate an AC supply for provision to the grid 10. The paralleling device 8 (which may be a switch) is provided to selectively connect the output of the inverter 4 to the grid 10.
The inverter 4 is used to convert electrical energy provided by the DC source 2 into the AC form required by the grid 10 by generating output currents that match the grid voltages in frequency, but that have amplitude and phase dependent on factors such as the amount of input power, the grid voltage level and the reactive power setpoint. Typically, the inverter 4 has a DC-bus capacitor bank connected to the inverter input circuit and an AC filter connected to the inverter output.
The physical connection of the inverter 4 to the grid 10 is carried out by the paralleling device 8. The paralleling device is typically instructed to connect the inverter 4 with the grid 10 during the inverter startup procedure.
Depending on the amplitude, phase and frequency deviation between the voltages on each side of the paralleling device 8, there may be an in-rush current at the moment the paralleling device 8 is closed. Another potential side-effect of improper inverter startup is a temporary reverse power flow from the grid 10 to the inverter 4 during the transition from off-grid to on-grid operation. Such reverse power flow can cause DC-bus capacitors to become overcharged.
In other scenarios, the inverter 4 can already be connected to the grid 10 (such that the paralleling device 8 is closed) but not actively producing any current or voltage for provision to the grid. This is typically referred to as the inverter being in a “coast” or “idle” mode of operation and the transition from such an idle mode to a running mode can also generate in-rush currents on the AC side and/or overvoltages on the DC-side.
A number of methods are known for connecting inverters (such as the inverter 4) to an AC grid (such as the grid 10) using a paralleling device.
In a first method, the paralleling device 8 is closed before the inverter 4 starts to generate output voltages or currents. This first method does not avoid the problem of in-rush currents outlined above. This method can perturb the grid voltages, cause interference to adjacent equipment and reduce the lifetime of inverter parts.
In a second method, the grid-side voltages are monitored and the paralleling device 8 is closed at a zero-crossing. The second method does not eliminate the in-rush currents due to the time the paralleling device takes to respond once zero-crossing is detected. In addition, this method is not applicable to multi-phase paralleling devices that close all phases at the same time.
In a third method, the voltages on the inverter side are actively measured and controlled to minimise the amplitude, phase and frequency deviation from the grid-side voltages. The third method required voltage sensors to be placed on both sides of the paralleling device, which are used within a dedicated voltage feedback control system specifically designed to make the inverter-side voltages converge to the grid-side voltages prior to closing the paralleling device. In addition to the cost and complexity of such a solution, since the voltage controller essentially operates with highly reactive components in the inverter output filter, the convergence time of the third method is typically many grid cycles and there is a risk of instability or the need to retune the controller due to ageing in the hardware components of the inverter.